A spark plug used for a combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in an axial direction, an insulator provided around the center electrode, a tubular metallic shell provided around the insulator, and a ground electrode whose proximal end portion is joined to a forward end portion of the metallic shell. The ground electrode is bent at its intermediate portion such that its distal end portion faces the center electrode, whereby a spark discharge gap is formed between a forward end portion of the center electrode and the distal end portion of the ground electrode.
Also, there has been known a technique of providing a tip formed of a noble metal alloy or the like on a portion of the ground electrode, which portion forms the spark discharge gap, to thereby improve durability and ignition performance. In general, the tip is joined to the ground electrode by a fusion portion which is formed by resistance welding or laser welding and which is composed of a metal which forms the ground electrode and a metal which forms the tip (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2007-87969, “Patent Document 1”).
Further, there has been proposed a technique of forming the ground electrode by using an outer layer, and an inner layer which is provided inside the outer layer and which is formed of a metal which has better thermal conductivity than the metal which forms the outer layer (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2001-351761, “Patent Document 2”). This technique makes it possible to quickly conduct the heat of the tip toward the metallic shell side through the inner layer, to thereby improve the corrosion resistance of the tip.
Incidentally, the tip is joined to the ground electrode by the fusion portion as described above, and the fusion portion is generally lower in thermal conductivity than the ground electrode. Therefore, in the case where the heat of the tip is conducted toward the inner layer side through the fusion portion, there arises a possibility that the heat of the tip cannot be conducted to a sufficient degree. In order to overcome such a drawback, there has been proposed a technique of bringing the tip into contact with the inner layer so as to cause the heat of the tip to flow directly to the inner layer without passing through the fusion portion (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-135783, “Patent Document 3”).
However, the amount by which the tip is intruded into the ground electrode must be increased so as to bring the tip into contact with the inner layer. Therefore, the tip is formed to be relatively long and have a large volume. In such a case, the amount of heat that the tip receives increases, and the heat of the tip may fail to be conducted sufficiently despite the tip being brought into contact with the inner layer.
The present invention has been accomplished in view the above-described problem, and its object is to provide a spark plug which can efficiently conduct the heat of the tip to the inner layer to thereby improve the corrosion resistance of the tip more reliably.